A diffraction grating is a series of parallel fine lines, slits or grooves on a substrate that diffract light passing through the grating. The grating period is the number of lines per unit length.
Presently, diffraction gratings are mainly constructed from mechanical means. These gratings have a fixed grating period which limits the ability to modify the diffraction of a light beam.
It is possible for liquid crystals to diffract light. Diffraction occurs whenever the orientation of liquid crystal molecules, described by the liquid crystal director, is periodically modulated in space. Spatial modulation of the director implies spatial modulation of the effective refractive index so that the liquid crystal serves as the phase diffraction grating. Chiral liquid crystals, such as cholesteric liquid crystals, are well known examples of media with periodically modulated (twisted) director fields. The cholesteric liquid crystal can be formed by chiral molecules or can be obtained by adding chiral molecules to a non-chiral nematic liquid crystal matrix.
When the liquid crystal is confined between two flat transparent plates and the director modulations are in the plane of the bounding plates (cell wall structures), the diffraction is controlled by the light wavelength, .lambda., the grating periodicity, .LAMBDA., and the thickness, d, of the liquid crystal layer (which is the distance between the bounding plates). The condition Q=(2.pi..lambda.d)/n.LAMBDA..sup.2, where n is the spatially averaged refractive index of the diffractive medium, defines Raman-Nath(RN) and Bragg regimes of diffraction. A thick grating, with Q&gt;1, corresponds to Bragg diffraction and can produce a strong diffraction maximum when the incident angle satisfies the phase-matching condition. A thin grating, with Q&lt;1, corresponds to RN diffraction. For normal incidence, the directions of the diffraction orders, m (where m=0; .+-.1; .+-.2; . . . ) are .theta..sub.m =arcsin(m.lambda./.LAMBDA.).
What is needed in the art is a diffraction grating that can be produced by a non-mechanical means, and in which the period of the diffraction grating can be changed by adjusting an electrical field.
It is therefore an object of the invention to provide a diffraction grating wherein the period of the diffraction grating is varied by an applied electric field.
It is another object of the invention to provide a diffraction grating with in-plane uniformity of the modulated cholesteric structure.